In recent years, the provision of broadband connections for customers of telecommunications companies has developed into a subject of decisive importance. To achieve maximum utilization from their infrastructure of already existing IP (Internet Protocol) based communications network, service providers are transitioning into offering a plurality of different services, such as conventional Internet access (IP data), Voice over IP (VoIP), Broadcast TV (IPTV), or Video-on-Demand (VoD), which drastically increases the demand for bandwidth placed on the existing infrastructure. This can have the result that the actually available bandwidth is no longer sufficient, and simple, cost-effective solutions must be found for increasing the bandwidth. Here, upgrading or replacing the physical transmission routes, especially cables, satellite transmission routes, and directional radio routes is usually ruled out.
FIG. 1 shows schematically a scenario in which, starting from a head end, a service provider couples broadcast traffic, which can comprise one or more broadcast data streams, and unicast traffic, which can comprise one or more unicast data streams, into a transport network (backhaul), especially an Ethernet, via a Broadband Routing and Access Server (BRAS). In addition to Unicast data generated by the service provider itself, external unicast data streams, e.g., VoIP data streams of subscribers from different networks or IP data streams of other service providers or subscribers from different networks, can also be fed to the BRAS. The unicast data streams generated by the service provider itself can involve, e.g., VoD programs or the like.
At this point it should be mentioned that a data stream does not necessarily have to exist as a separation physical data stream. Several different (logical) data streams can be combined to form a single physical data stream, for example, through packet-multiplexed, time division-multiplexed, or wavelength division-multiplexed techniques, which is then represented by a corresponding signal. A data stream, however, is assigned to a certain source port, where it is coupled into the transport network, which is preferably constructed as a high-speed transmission network. In addition, a unicast data stream is also assigned to a certain target port, where it is decoupled from the transport network. Obviously, each data stream can consist of several or a plurality of sub-data streams, which are similarly each assigned to a target port and/or a source port in the scope of a protocol stack.
FIG. 1 shows a realization, in which a service provider generates at the head end a broadcast data stream, which includes, for example, several TV programs (indicated in FIG. 1 by the box with the satellite antenna), and a unicast data stream for providing a VoD service (indicated in FIG. 1 by the box with the film roll). These data streams are fed from the named data sources to a broadcast server or a VoD server and coupled by these into the transport network via the BRAS. Here, the broadcast and unicast data streams are typically combined into one data stream. This data stream can involve, e.g., a Gigabit Ethernet data stream, in which each frame contains both broadcast data of the broadcast data stream and also unicast data of the unicast data stream.
The data stream transmitted via the transport network is decoupled at a network node KN1, KN2 of the transport network, which corresponds to the target port of the data stream, and is broken down into sub-data streams, which are fed to the subscribers. The splitting of the data stream transmitted via the transport network can be realized, e.g., via a DSLAM (Digital Subscriber Line Access Multiplexer), to which, on the local side, e.g., 500 subscribers can be connected. The selection of the TV program and the splitting of the received signal into the sub-signals for the different end devices and the combining of the sub-signals can be realized on the subscriber side by means of a set-top box (STP).
For increasing the data transmission capacity or the bandwidth of the transport network, it is known to generate at the head end several data streams, which each contain broadcast and unicast data and which combine these through a time division-multiplexing method in the transport network into a single physical data stream between the head end and local loop.
To connect several, usually spatially separated local loop network nodes to the head end using one and the same fiber pair of an optical transport network, the known technology of wavelength division multiplexing is used. Here, one or more dedicated optical wavelengths correspond to a defined local loop network node. The coupling and decoupling of the wavelengths is realized by so-called optical add/drop multiplexers (OADM). For a certain network node, if a correspondingly high bandwidth is needed, then it is obviously also possible to terminate two optical channels, i.e., two wavelengths (more precisely: intermediate wavelengths) in this network node.
This known method is shown in FIG. 2, wherein a time division-multiplexing/demultiplexing unit 1 with two local-side connection ports S1L and S2L is provided for combining the two head end-side data streams shown in the embodiment in FIG. 2. In the schematic representation in FIG. 1, this unit can be arranged in the downstream direction after the BRAS or integrated into this server. The BRAS in FIG. 1 can be constructed so that it generates the two data streams, which each contain the same broadcast data stream B and a unicast data stream U1 or U2. Each of the two data streams is assigned to a certain source port, which corresponds to the connection port S1L or S2L of the time division-multiplexing/demultiplexing unit 1. The time division-multiplexing/demultiplexing unit 1 combines the two data streams at the connection ports S1L and S2L into a single data stream, which is coupled into the transport network at the remote-side connection port SR via an add/drop multiplexer, which is constructed as an optical add/drop multiplexer (OADM). The sub-data streams combined into one physical data stream are, however, still assigned to the relevant target port.
In this way, each sub-data stream can be decoupled at that network node containing the target port. FIG. 2 shows a situation in which both sub-data streams are assigned to a target port that is assigned to the same network node. Thus, the entire time division-multiplexed signal containing both sub-data streams is decoupled at the same network node and split into the two physical sub-data streams by means of another time division-multiplexing/demultiplexing unit 3, which is connected in turn in series after an OADM with its remote-side connection port PR. The sub-data streams connect, in turn, to the local-side connection port P1L, P2L of the time division-multiplexing/demultiplexing unit 3, to which the target ports of the sub-data streams are also assigned.
As is visible from FIG. 2, the two sub-data streams are combined into a single physical data stream for transmission via the transport network, such that the data transmission rate is essentially doubled and the sub-signals are “interlaced” by a time division-multiplexing method while maintaining their structure, wherein as before each sub-data stream contains all of the information consisting of U1 and B or U2 and B.
For reasons of a simpler representation, the connection ports of the time division-multiplexing/demultiplexing units 1 and 3 are shown as bidirectional ports. Obviously, however, a unidirectional receive port and a unidirectional transport port can also be provided for each bidirectional port.
This known method produces a doubling of the transmission capacity of the transport network. However, twice the bandwidth within the transport network is also required.
The embodiment shown in FIG. 1 involves for the sub-data streams Gigabit Ethernet signals, so that the transport network must be in the position to transmit a time division-multiplexed signal with a data rate of 2 Gbit/s.
Obviously, this known method can also be expanded, wherein at least three sub-data streams are combined into one time division-multiplexed signal to be transmitted via the transport network. This leads, however, to a corresponding multiple increase in the bandwidth requirement with reference to the transport network.
As a solution, it has been proposed, in addition to the use of the time division-multiplexing method, to use a wavelength division-multiplexing method, wherein several time division-multiplexed signals could be transmitted each with a different carrier frequency or carrier wavelength via the transport network using the previously explained means and methods. In particular, for optical transmission via the transport network, the otherwise existing large bandwidth of optical fiber transmission network could be better utilized in this way.
For optical transmission networks for IP-based data traffic, as previously explained, typically, the entire time division-multiplexed signal, which is contained in an optical channel, is terminated at a node and split by means of a time division-multiplexing/demultiplexing unit into the individual sub-data streams or the individual sub-data streams are combined into the complete time division-multiplexed signal by means of the time division-multiplexing/demultiplexing unit. Thus, at a network node, either the full bandwidth of a complete time division-multiplexed signal is available, which is split by means of the downstream-connected DSLAM and a corresponding protocol or a corresponding protocol stack to the individual subscribers. Here, the DSLAM, considered in the OSI layer model, can take over the function of a layer 2 and/or layer 3 switch or router, so that the decision regarding which part of the IP traffic terminates at the node and which is assigned to certain end users or is passed through to a different node is made in the DSLAM (or also suitable equipment connected after the DSLAM).
In this way, the flexibility in the design of more complex transmission networks is limited, or relatively complicated network equipment (especially complex and thus expensive DSLAMs, switches, routers, etc.) is required.